The primary aim of this proposal is to quantitate the influence of diethylcarbamazine (DEC) on the adherence and phagocytic activity of leucocytes on Onchocerca volvulus microfilariae (O.v. mf). This will be studied in vitro using an incubation medium of saline phosphate and blood serum. O.v. mf added to this medium have been found to remain intact for many hours with no leucocytic adherence to mf; in the presence of DEC leucocytic adherence to mf and death of mf occur. The induction of leucocytic adherence to O.v. mf by DEC will be studied using sera from non and infested onchocerciasis patients. Preliminary evidence that this phenomenon may be used to identify patients with active O.v. mf infestation will be examined with the aim of developing a simple and accurate serological diagnostic procedure. This is an urgent need in view of the unreliability of the skin snip procedure in patients with mild onchocerciasis where the majority or all of the skin snips may be O.v. mf negative (see Langham, ref. 1). The second aspect of the proposal deals with the elucidation of the microfilaricidal action of DEC and aims at improving the efficacy and safety of DEC in treatment of onchocerciasis especially as this relates to the prevention of blindness. Ultrastructural studies have shown that the microfilaricidal action of DEC may be associated with an internal dissolution of organelles with little, if any, localized cellular response or it may be associated with marked localized edema with dense infiltration of leucocytes. This latter reaction is associated with marked phagocytosis including lysosomal activity which may cause tissue damage. The present study aims at defining the parameters controlling the two patterns of microfilaricidal action by DEC. Recent evidence that the diagnosis of onchocerciasis patients with O.v. mf negative skin snips may be made by the presence of a characteristic dermatitis and the isolation of mf from localized papules following treatment with DEC will be extended to determine the potential accuracy of this diagnostic approach (see Langham and Richardson, ref. 2).